Czar Nicholas II of Russia
Nicholas was the last Czar of the Russian empire, succeeded his father in 1894. Nicholas and his wife Alexandra of Hesse were coroneted as ruler in 1896. As ruler he instructed the building of the Trans-Siberian railroad. He took charge of the military during World War I and left his wife and her advisor to internal social affairs. Years of discontent in policies in Russia and led the Russian revolution and in 1918 Nicholas and his family were executed. It was under his authority that Russia aligned itself with Serbia that began a cascade toward war. With his defeat, Russia could no longer support the war and retracted.
Kaiser Wilhelm II of Germany
He was the grandson of William I and the eldest son of Frederick III. He built up the German military to rival the British in 1900. The British then began to increase the number and size of their ships, and other European powers followed their example. Armies also were enormously increased as the nations became engaged in an armaments race. Wilhelm was quite obsessed with military matters and thus proved ineffective as the kaiser of Germany. His dream of world power continued even after WWI which shows the extend to which this particular ruler might have stirred up or prolonged the world war.
Premier Georges Clemenceau of France
Georges Clemenceau was the ruler of France during World War One. He supported the idea of war. In fact, he didn't believe in surrender, and the excessive pride would thus drive the war. His goal was to expose the inefficiencies of the French military, and thus a war was needed to do that. This sense of nationalism by the key leader would cause a giddiness for war that caused France to so quickly respond to Russia's call for help after the assassination of the archduke.
Prime Minister David Lloyd George of Great Britain
Prime minister during the war, he at first didn't feel inclined in hopping into the war. After the Belgian neutrality was violated by the German attack, he supported the war, and after he became Prime Minister in 1916 he supported the war effort and governed it in such a way that England was somewhat successful.
President Woodrow Wilson of the United States
Woodrow Wilson was the President of the United States during all of World War One. His idea of the war was that the United States should remain neutral, and he thus didn't partake in much action. Even after the Lusitania was sunk, which carried 124 Americans, the US remained neutral for two more years. In contributing to the war, it happened that the Lusitania was smuggling weaponry to Britain which only inflamed the Germans. After the Russian Revolution, in which Russia became somewhat democratic, Wilson felt that in aiding Russia against Germany that there would be no conflict in governmental type, saying "The world must be made safe for democracy."
Revolutionary Vladimir Lenin of Russia
Vladimir Lenin was the man most responsible for the success of the Russian Revolution. His ideas aligned with those of Karl Marx's. He found the people of Russia in poverty, and he figured that the best way to solve the problem was to submit Russia into communism. Though his success, Russia became the Soviet Union, and the US felt more comfortable in joining them against Germany.